1. Field of the Invention
The present invention relates to an air-oil cooled engine, and to a vehicle incorporating the air-oil cooled engine. More particularly, the present invention relates to a four-cycle air-oil cooled engine including a cylinder head having a spark plug mounting hole formed therein and a ring-like groove surrounding the spark plug mounting hole. The spark plug mounting hole is adapted to receive a spark plug therein. The engine also includes a cover member for closing an opening end of the groove defining an oil jacket disposed around a periphery of the spark plug mounting hole; and a gasket disposed between the cylinder head and the cover member secured to the cylinder head; and to a vehicle (motorcycle) incorporating the same.
2. Description of the Background Art
There are a number of known four-cycle air-oil cooled engines. For example, the Japanese Utility Model Laid-open No. Hei 2-22621 discloses a four-cycle air-oil cooled engine, in which an annular groove provided on the periphery of a spark plug is covered with a cover member provided on the cylinder head for defining an oil jacket on the periphery of a spark plug mounting hole.
The four-cycle air-oil cooled engine disclosed in Japanese Utility Model Laid-open No. Hei 2-22621 is configured as discussed herein. The cylinder head is provided with cylindrical columnar portions, which are coaxial with corresponding respective spark plug mounting holes. The cover member is arranged such that a first annular gasket, disposed inward of the groove, is interposed between the upper surface of the cylinder head and the cover member, and a second annular gasket, disposed outward of the groove, is interposed between the inner circumferential surface of the columnar portion and the cover member.
Also in the four-cycle air-oil cooled engine disclosed in Japanese Utility Model Laid-open No. Hei 2-22621, the spark plug threadably engaged with the spark plug mounting hole, and the cover member is sandwiched between the cylinder head and the spark plug. Thus, two gaskets are needed, which increases the number of component parts required, and makes sealing surfaces complicated. Such a complicated sealing arrangement makes it difficult to ensure sealing performance.
In addition, during the assembly of the cover member, it is difficult to assemble the two gaskets, one of which is interposed between the upper surface of the cylinder head and the cover member, and the other of which is interposed between the inner circumferential surface of the columnar portion and the cover member. This requires improved assembly performance.
The present invention has been made to overcome such drawbacks of existing four-cycle air-oil cooled engines. Accordingly, it is one of the objects of the present invention to provide a four-cycle air-oil cooled engine that improves sealing performance and enhances assembly performance, while simplifying a sealing structure between a cylinder head and a cover member used to define an oil jacket.